Electric motor follow-up system



y 1947- F. l... MOSELY EY ET AL ELECTRIC IQTOR FOLLOI-UP SYSTBI 2 Shuts-Shoot 1 and larch 6, 1941 INVENTORS FRANCIS ERIC J L MOSELEY, ISBISTER.

HAYES B lz/ 5mm sen Hanan-fella. 1

Patented July 29, 1947 ELECTRIC MOTOR FOLLOW-UP SYSTEM Francis L. Moseley, Chevy Chase, Md.l Eric J.

Isbister, Brooklyn, N. Y., and Hayes B. Steinliauser, Jersey City, N. 1., assignors to Sperry Gyroscope Company,'lne., Brooklyn, N. Y., a corporation of New York Application March 8, 1941, Serial No. 382,020

11 Claims. 1

This invention relates, generally. to electrical control systems and the invention has reference, more particularly, to a novel remote electrical positional control system of the general type disclosed in the copending application Isbister and Moseley, Serial No. 304,895, illed November 17, 1939, wherein a tumable controlling object is arranged to operate through suitable electrically operable means to cause a controlled object to turn in substantial synchronism with the controlling object, said electrically operable means employing a pair of signal voltage transmitters or synchronous generators operating in 'diilerent speed ratios, i. e., a fine or high speed transmltter and a coarse or low speed transmitter, corresponding receivers being employed adjacent the controlled object and operating through novel servo mechanism for determining the motion of the controlled object.

In order to obtain great accuracy oi reproduction' by the controlled object, 1. e., substantial synchronism of the objects, it is necessary to employ a fine or high speed signal voltagev transmitter and connected receiver arrangement. This high speed arrangement may operate in a ratio, for example, of 36:1, providing one channel of the synchro-transmission system. The other channel of the transmission system is connected in a 1:1 ratio inasmuch as the objects may get out or step any amount from zero to 180 in either direction'due, for example. to the turning of the controlling object. such as a handwheel, when the system is not energized.

The principal object 01' the present invention is to provide a novel positional control system employing high and low speed transmitter-receiver arrangements, a remote servo system controlled therefrom for operating a controlled object in synchronism with the low speed transmitter, the power supply for the transmitter-receiver arrangements being independent of that provided 'for the remote servo system, improved circuit means being provided for converting the output of the transmitter-receiver arrangements into a reversible phase, variable magnitude signal for use in controlling the servo system.

Another object of the present invention lies in the provision of a remote positional control system wherein means are provided for isolating ground currents for preventing these currents from circulating in a manner which would tend to change the bias oi the tubes employed in the servo system.

Still another object of the present invention is to provide simplified means for eliminating 180 ambiguity of the controlled object with respect to the operation of the transmitter equipment, 1. e., simplified means being provided or preventing the controlled object from coming to rest 180 out of phase with the controlling obiect.

Still another object of the present invention is to provide a remote positional control system having improved means for producing not only a displacement signal, but also rate signals, i. e., velocity and acceleration, for use in conjunction with the displacement signal to eilect antihunt control of the servo system and controlled object.

Other objects and advantages will become apparent from the specification, taken in connection with the accompanying drawings wherein the invention is embodied in physical form.

In the drawings,

Fig. l is a wiring diagram illustrating one form of the novel positional control system of this invention.

Fig. 2 illustrates a portion of the circuit 01' Fig. 1, somewhat modified.

Fig. 3 is a diagram illustrating the potentials involved in the output of the transmitter-receiver circuits 01' Fig. 1.

Similar characters of reference are used in all oi. the above figures to indicate corresponding parts.

Referring now to Fig. 1 of the drawings, the controlling object i is illustrated as a handwheel, although the same might .be any small power-turned object such as a telescope, the angular position of which is to be accurately and immediately repeated by a controlled object 2, which may be a ponderable tumable object. The high and low speed transmitting devices 3 and l and their remote connected receiving devices 5 and 6, constituting a synchronizing circuit, may be of any suitable type. Preferably, these devices are of the A. C, type in which the transmitting and receiving devices are .similar in construction and are each provided with a polycircuit armature winding 1 physically similar to a three-phase star-connected armature winding. This armature winding may be mounted on the stator, while the field winding 8 is on the rotor. In connecting each receiving device to its respective transmitting device, three conductors are used by means of which like oints of the two armature windings l of the devices are interconnected. The field windings 8 of the transmltters are connected to a suitable source of A. C. supply led through the single phase leads 3 9, which may be supplied-from a local source, if desired. Field winding 8 of the high speed transmitter 3 is comiected to handwheel I through step-up gearin i0, whereas field winding 3 of the low speed transmitter 4 is connected to turn at the same rate as the handwheel l.

Transmitter field windings 8 produce in their respective armature windings I, alternating magnetic fields having positions in space determined by the relative positions of windings l with respect to windings 8. Any rotation of the transmitter field windings or rotors 8 caused by angular movement of the handwheel produces a corresponding angular shifting of the axes of the magnetic fields of their respective armature windings, resulting in a corresponding shifting of the fields of the armature windings 1 of the receivers and 6, thereby tending to cause field windings 8 of the receivers to follow up such shift, but since these field windings are mechanically connected by the mechanical follow-up transmission [3 to the controlled object 2, alternating E. M. F.s are induced in the field windings 8 of receivers i and 6, the values of which E. M. F.s are substantially proportional to the angular displacement between the handwheel I and object 2, i. e., for small angular displacements, and the phase of which E. M. F.s reverse witha reversal in the direction of turning of the handwheel.

The A. C. signal voltage induced in field winding 8 of the high speed or fine receiver 5 is applied across the primary of a transformer ll having a resistor l5 in series therewith. The secondary of transformer i4 is connected to excite a neon tube l6, whereby the latter serves to limit the effective voltage output of the high speed receiver.

The thusly limited voltage of the high speedor fine receiver 5 is added in series with the signal voltage output of the low speed or coarse receiver 6 and with a fixed anti-stick-ofi voltage supplied from the secondary of a transformer I! that has its primary excited from the supply 9. This fixed voltage exists between the center of; the secondary winding of transformer l1 and the common connection point [8 of two resistors 19 and 20 connected to the ends of the secondary of transformer l1 and by lead to the rotor 8 of the low speed receiver 6. The summation voltage consisting of the limited fine receiver voltage, the low speed receiver voltage and the fixed antistick-off voltage connected in series, is supplied by a lead 2| to unlike ends of the primaries of input transformers 22 and 23, the circuit being completed via leads 24 and 24.

The transformer I! also supplies an A. C. reference voltage or signal through leads 24 and 24' passing in series through the primaries of transformers 22 and 23. The secondardies of transformers 22 and 23 are connected to full wave rectifiers 26 and 21, respectively, having output resistors 28 and 29. The full wave rectifiers or double modulators 26 and 21 act as an isolation circuit in that they serve to separate the alternating current of the synchro-transmission system from the alternating current supply of the servo circuit. This enables the signal system fed from A. C. supply leads 9 to be supplied from a power source which is not necessarily interlocked with the power source 30, 3|, 32 supplying the servo end of the system. The D. C. outputs of the full wave rectifiers or double modulators 26 and 2'! are supplied to a double triode vacuum tube 33 serving as a modulator and amplifier. The plate cathode circuit of tube 33 is supplied from a transformer 34 energized from leads and 32 of the three phase leads 3|, 3|, 32. Plate current from transformer 34 divides in a center tapped choke 35 passing to the plates oftube 33. Choke 35, cooperating. with tube 33 and a rate taking transformer 36, serves to produce a reversible phase, variable magnitude signal comprising not only displacement but also rate terms, as will further appear, which signal is fed to the grids of grid controlled rectifiers 31 and 38 having output transformers 39 and ll, the secondaries of which are connected respectively to the two stator windings 4| and 42 of the two-phase induction motorv 43 driving the controlled object 2.

The operation of the system of the present in- .vention will be better understood by referring to the diagram of Fig. 3. The voltages shown in this figure are the R. M. S. (meter) values of the various voltages and are plotted against the angular position of the controlled object with respect to the controlling object. Voltages plotted below the horizontal or X axis are to be understood to be 180 out of time phase with those plotted above this axis If the summation voltage reverses its time phase, the controlled object reverses its direction of rotation. Curve V1 shows the open circuit voltage across the receiver rotor 8 of the 36:1 transmitter-receiver device. It will be seen that this voltage has a zero every five degrees. The phase relations are such, however, that so far as this voltage alone is concerned, the controlled object will synchronize only at the even zero points, i. e., at 0, 10, 20.

Curve V: shows the open circuit voltage of the 36:1 receiver rotor 8 as limited by the action of the neon tube Hi. This is the voltage that appears across the primary of transformer l4. Neon tube l6 appears as an open circuit across the secondary of transformer H until the ignition.

voltage of this tube is reached, and as the open circuit primary impedance of transformer H is high, substantially the open circuit output of the 36:1 Selsyn appears across this point, but when the tube fires, it practically short-circuits the secondary of transformer II and this low impedance is reflected to the primary and, in conjunction with resistance 15, prevents the signal from increasing any further. It can be seen that the limited signal rises from zero with the same slope as the unlimited signal and is then sharply cut off, as shown in curve V2, at a low value. The ratio of transformer ll must be such that the peak value of the limited signal is always less than the value of the 1:1 signal at :5 from zero. Curve V3 shows the open circuit voltage of the 1:1 transmitter-receiver device. This curve has only two zero points per revolution of the controlled object, and the phase relations are such that the object is always driven to the 0 point.

If the limited 36:1 signal and the 1:1 signal wer added together and applied to the control of the object 2, then we would have only two synchronous points, namely, zero and-180. It will voltage Vs gives the 1:1 signal voltage a zero at 0 and at as shown by curve V4, and adds in the proper phase relation with the limited 36:1 signal to give only one point of synchronization for the controlled object, namely, the point. The zero signal point at 175, shown by curve V of Fig. 3, is an unstable zero and our attempt to hold the controlled object at this point is like trying to balance a needle on its point, which is practically impossible since the controlled object tends to move from thislpoint and in a direction dependent upon the resultant voltage.

If the sum of the fixed or anti-stick ed! voltage plus the limited 362-1 and the 121- .ioltage is equal to zero, then the reference voltage or signal supplied from the secondary of transformer II will cause equal currents to flow through the primaries of transformers 22 and 23, this current flowing by way of leads 24 and 24'. cause equal 'D. C. voltages to appear across the output resistors 28 and '23 of the rectiflers 26 and 21. As connected, these equal D. .C. voltages will balance to zero across input resistors 44 and- 46 of tube 33 so thatthere will be no net voltage across the grids of triode33.

If, however, the combin'ed-Selsyn and antistick-off voltage, 1. e., curve V5 in Fig; 3, is not zero, it will add to the reference voltage of transformer il in such a way that it will either increase This will.

make the upper end of choke 36 negative with respect to its lower end. If the polarity of the D. C.

. voltage on the grids of tube 33 should be reversed due to reversal'of the direction of rotation of 5 handwheell, th'en in that case the lower end of choke 35 becomes more negative than its upper end. As this current is pulsating D. C.,- the condensers 43 and 43, connected in series across choke 36, serve to convert this current into a lo fair wave form :A. C. with a steady D. C. com- 16 placement signal, passes through transformer 36 and its associated circuits to the grids of the grid controlled rectiflers 31 and '33. In other words, this A. C. signal supplied upon the grids of tubes 31, and 33 is substantiallyproportional 20 tov th relative displacement between the controlled object 2 and the controlling object I.

If the signal on the grids of vacuum tube 33 is constant, corresponding to a fixed relative displacement, the net D. C. component of the plate current acrosschoke is constant and does not the current flowing through the primary of transone transformer and subtractive in the other with respect to the reference voltage] or signal. Thus,

assuming that lead 21 is positive at the time lead 24 is positive, then the combined signal voltage will oppose the reference voltage in the primary of transformerv 22 and will add with the reference voltage in the primary of transformer 23 so that ther will result unequal D. C. voltages across output resistors 26 and 29 of rectifier tubes 26 and 21 and a net D. C. signal voltage willappear across the grids of the vacuum tube 33, the polarity of which voltage will reverse with a reversal in direction of turning the handwheel I. The tuned circuit 46 serves to filter'the output of the rectifiers 26 and 21. This tuned circuit is preferably tuned to the double frequency output of the rectifiers. The resistors .44 andliprovidea center ta'p connected to the filaments of tube. 33 to provide a grid return. This center tap is grounded, as shown at 41, forpurposes which will v notonly with a displacement signal but also-with be further explained.

The vacuum tube 33 serves as a modulator and amplifier and is shown as supplied from the transformer 34, preferably with 60 cycleA. C. for its plate voltage. The plates of this tube are connected in parallel so far as this supply is concerned, whereby the tube is operative only during half of each cycle. If there is no signal upon the grids of this tube, equal but opposite half wave rectified plate currents flow through each half of the choke 35. Assumingthat a' relative displacement takes place betweenthe objects I and 2 so as to make the polarity of the D. C. 'grid for example, is positive with respect to the lower grid thereof, more current will flow through the upper half of choke 35 than will flow through the lower half thereof. This unbalanced current will pass through transformer 38. displacement signal'is changing, causing the D. C.

' signal on the "grids of tube 33 to be changing, the

D. C. plate current across choke 35 correspondingly changes. The voltage across the D. C. resistance of choke 35 is proportional to the current through it and, therefore, to the displacement between'the controlled object and the con- 35 trolling object. The voltage induced across the inductance of choke 35, however, is proportional to the rate of change of current through it, i. e.,

" to the rate of change of displacement, whichis velocity. These two- D. C. component's, corresponding to displacement and the first derivative thereof, are fed to the primary of transformer 36 and if these two voltages are changing, the primary inductance of the transformer takes derivatives of these components so that voltages 5 appear across the secondary of this transformer displacement of the objects is not changing,

, neither of these voltages will be present, and if that are substantially proportional to theQfir-st and secondderivatives of ,the displacement with respect to time, i. e., to the relative velocity and acceleration between the objects. /If the relative the relative displacement is changing uniformly, only one, the first derivative of displacement with respect to-time, will appear, but if the relative displacement of the objects is changing with acceleration or deceleration, both voltages will appear. Thus, the tubes 31 and 38 are controlled ,velocity and acceleration signal where these termsobtain; as where the handwheel l is being moved with acceleration. The use of these three 35 Condensers 48 and 48 and also condenser 50,

this last condenser being connected across the primary of transformer 36, in addition to improv- 'ing the wave form of the A. C. component of the signal, act by reducing this A. C. or displacement 79 component somewhat to relatively increase the signal such that the upper grid of this'tube 33, I

, 54 serve a similar purpose in that they tend to attenuate the A. C. displacement signal while enhanclng the rate signals. The resistors 53 and 64 If, however, the

also supply a low impedance path for the high frequency currents generated during the operation of grid glow tubes 31 and 38. The tubes 31 and 38 are controlled by the resultant signal voltage appearing across the secondary or transformer 36, which acts to shift the phase of the net signal from the grid to cathode. This is accomplished by feeding the grid of tube 31 with the component of the signal voltage across resistor 53 taken in series with an A. C. bias voltage supplied from a transformer 55 energized from threephase leads 30, 3| and 32. Also, the grid of tube 38 is fed with that portion of the resultant signal voltage across resistor 54 added in serie with the A. C. bias voltage from the secondary of transformer 55.

The combined signal voltage output of transformer 36 is maintained in phase with the plate voltage supplied by transformer 55 to the plates of tubes 31 and 38. The A. C. bias voltage from the secondary of transformer 55 lags the plate voltage of tubes 31 and 38 by approximately 120. As the signal voltage across resistor 53 increases with such phase relation that it adds to the A. C.

. bias voltage from transformer 55, the net signal on the grid of tube 31 advances in phase, causing the plate current of tube 31 to progressively increase. At the same time the voltage across resistor 54 is of such phase that it subtracts from the A. C. bias voltage supplied by transformer 55, so that the net signal voltage on the grid of tube 38 is retarded in phase, thereby progressively reducing the plate current of tube 38. When the phase of the signal voltage reverses, as when the handwheel l is turned in the reverse direction, the phase of the voltage on the grids of tubes 31 and 38 is shifted in opposite direction, resulting in an increase of the plate current of tube 38 and a reduction of the plate current of tube 31.

The plate currents serve, in efiect, to short the secondaries of transformers 33 and 40 in a manner similar to that described in connection with the above mentioned application Ser. No. 304,895. Thus, when tube 31 is passing current, tr former 39 is shorted in effect, so that windirz il of motor 43 takes a current that is in phase with that of leads 3| and 32, whereas the current passing through winding 42 will be leading, since this current flows through the condenser 51, causing motor 43 to operate in one direction. On the other hand, when tube 38 is passing current, transformer 48 is shorted in effect, so that winding 42 takes the current in phase with leads 3| and 32 and winding 4! takes a leading current which flows through condenser 51, causing motor 43 to turn in the opposite direction. Since the change in the impedances of transformers 33 and 48 is substantially proportional to the signal voltage, the speed of the motor is controlled substantially proportionally to the signal voltage. Resistor 58 serves to absorb some of the transients that occur when tubes 31 and 38 are firing.

The transformer 38, in addition to acting as a differentiator, serves also to prevent ground currents from circulating and altering the bias of grid controlled rectifier tubes 31 and 38. The core of this transformer is shown grounded so as to reduce the capacity coupling between its windings to as low a value as practicable. The cores of the other transformers are preferably grounded also. In the circuit of the above mentioned application Ser. No. 304,895, capacity coupling was used between the tube corresponding to tube 33 and the grid circuits of the connected grid glow tubes. In that case. the oathode of the tube corresponding to tube 33 was not grounded, so that a small A. C. current was thus allowed to flow from the plate circuit of the tube corresponding to tube 33, through the capacity coupling to the grid circuits of the thyratron tubes, and thence across their input resistors through the bias voltage supply transformer to ground, and then through the capacity to ground of the plate supply transformer of the tube corresponding to tube 33 back to the plate supply of said tube. This current was caused to flow by the high plate voltage from the transformer supplying the amplifier-modulator tube corresponding to tube 33, and which current, acting across the input resistors 01 the grid glow tubes, produced a voltage drop, thereby changing the bias of these tubes and causing improper operation. Any change in the bias condition of these tubes results in change in sensitivity of the amplifier and will cause it to be very insensitive, or vice versa. These capacitative changes are functions of temperature and time and are eliminated in the present circuit since the transformer 36 effectively blocks such circulating currents from reaching the tubes 31 and 38, and since any capacitive circulating current from transformer 34 will pass through transiormer 36 to ground and by way of ground 41 directly back to transformer 34.

Instead of employing the two resistors l9 and 28 connected with the ends of the secondary oi transformer I1 and connected together at point l8 for supplying the anti-stick-oif voltage, a tap 20 may be provided on the secondary oftransformer I1, as shown on Fig. 2 of the drawings, this tap being connected to lead 25.

As many changes could be made in the above construction and. many apparently widely dinerent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a. remote positional control system, a plurality of objects arranged to move in synchronism, synchronizing means interconnecting said objects, said synchronizing 'means comprising a synchronizing circuit including transmitter and repeater means for setting up an alternating reversible phase potential signal responsive to the departure oi the objects from synchronism, an alternating current supply for said synchronizing circuit, an isolation circuit including modulating means fed from said synchronizing circuit for converting the alternating reversible phase potential signal output thereof into a reversible direct current potential signal, a second source of alternating current supply, thermionic tube means fed from said second source and connected for receiving said reversible direct current signal and for converting the same into a composite signal including a reversible phase alternating current signal component and a D. C. surge signal component, a servo motor mechanism including a motor for driving said controlled object, said thermionic tube means including means for receiving said composite signal and for controlling said motor, and a follow-up connection from said servo motor mechanism to said synchronizing circuit.

2. 111a positional follow-up control wherein a reversible motor drives a. system into angular correspondence with a synchro transmitter means,

synchro transmitter means, a driven system, driving means for said system including a reversible motor, receiver means connected to said trans-- mitter means for producing a reversible phase alternating voltage signal in accordance with relative displacement between said two interconnected means, a. follow-up connection between said motor and said receiving means in part controlling said relative displacement, means for producing an alternating reference voltage for combination with said reversible signal, dual modulating means connected for receiving the combined reversible signal voltage and the reference voltage, one of said modulating means receiving these voltages in additive relation while the other of said modulating means receives these voltages in subtractive relation, said dual modulating means serving to convert these combined voltages into a reversible polarity, direct current signal, and thermionic means connected for receiving said direct current signal including means for controlling the speed and direction of rotation of said reversible motor in accordance therewith.

3. In a positional controlsystem of the character described, a plurality of objects arranged to move in synchronism, synchronizing means interconnecting said objects including a reversible motor for maintaining said objects in synchronism, said synchronizing means also including high speed and low speed transmitter and connected receiver arrangements for setting up alternating potentials corresponding to the departure of said objects from synchronism, means for limiting the maximum potential output of said high speed transmitter-receiver arrangement, means for supplying an alternating potential signal of substantially constant value, means for combining said three signals, means for supplying an additional alternating reference signal for combination with said other signals, and means for impressing the resultant upon a thermionic tube circuit, the output of which controls the speed and direction of rotation of said reversible motor.

4. A positional control system of the character described comprising a plurality of objects arranged to move in synchronism, synchronizing means interconnecting said objects, said synchronizing means including high speed and low speedtransmitter and connected receiver arrangements for setting up alternating variable signal potentials responsive to the departure of said objects from synchronism, transformer means for supplying a fixed alternating potential. means connecting said fixed alternating potential in series with said variable signal potentials, thermionic rectifier means, said transformer means also supplying an additional altemating reference potential for combination with said other potentials for application to said thermionic rectifier means to produce a reversible direct current potential, thermionic tube means for receiving said reversible direct current potential and for reconverting the same into an alternating current potential, motive means controlled from said thermionic tube means for causing said objects to return to synchronism, and a follow-up connection between said motive means and said high and low speed receiver arrangements.

5. In a positional control system, the combination of controlling and controlled objects, a

motor for driving said controlled object, thermionic tube means for controlling said motor,

high speed and low speed transmitters operat- I ed from said controlling object, high speed and low speed receivers electrically connected to said transmitters, said receivers being arranged to be electrically connected to said thermionic tube means to'supply signal voltages thereto, voltage limiting means connected to the output of at least one of said receivers for limiting the signal voltage of the latter, means for supplying an alternating voltage in series with the output of said receivers to be applied to said thermionic tube means, said added alternating voltage serving to eliminate 180 degree ambiguity of said controlling and controlled objects, said last named means also servingto supply another alternating voltage'serving as a reference voltage for use in detecting the phase or said signal voltages, and a mechanical follow-up connection between said controlled object and said high and low sp ed receivers.

6. In a remote control system, a transmitterreceiver arrangement for producing a reversible phase alternating current signal, means for producing an alternating reference voltage, dual full wave rectiflers, said alternating current si nal being additive to said reference voltage for application to one of said rectiilers and being subtractive from said reference voltage for application to the other of said rectifiers, a pushoutputs of said grid glow tubes.

7. In a positional control system, a transmitterreceiver arrangement for producing a reversible phase alternating current signal, means for producing an alternating reference voltage, and dual full wave rectiflers, said alternating current signal being additive to said reference voltage for application to one of said rectifiers and being subtractive from said reference voltage for application to the other of said rectifiers, a push-pull amplifier having its grids connected to the outputs of said respective rectifiers, means for supplying alternating potential to the plate-cathode circuit ofsaid amplifier whereby said amplifier has an A. C. output responsive to the differential output of said rectifiers, said amplifier having impedance means in its plate circuits for producing surge potentials corresponding either to the displacement signal or a time derivative thereof, transformer means connected for receiving said alternating and surge voltages and for taking time I derivatives of the latter, dual grid glow tubes fed ing a reversible alternating signal from the rela-- tive displacement between the transmitter means and the driven system, means for producing a fixed alternating current signal, means for combining said two signals, means for producing an alternating reference signal, modulator means supplied with a version of said combined signals and said reference signal and acting to convert the same into a direct current signal, a second modulator for reconverting said direct current signal into a resultant alternating current signal, a thermionic tube circuit, means for impressing the resultant alternating current signal upon said thermionic tube circuit, the output of which controls the direction and speed or rotation of said reversible power motor, and a followup connection between said motor and said transmitter means.

9. In a positional control system of the character described, a high speed and a low speed transmitter-receiver arrangement, means for limiting the voltage output of said high speed transmitter-receiver arrangement, transformer means for supplying an anti-stick-oif voltage in series with the voltage outputs of said transmitterreceiver arrangements and for supplying a reference voltage to be combined with said tranbmitter-receiver and reference voltages, a modulator for receiving said combined voltages and for converting the same into a reversible D. C. signal, a second modulator for reconverting said reversible D. C, signal into an alternating reversible phase signal, grid controlled rectifier means, transformer means receiving the output of said second modulator for applying. the alternating current signal and one or more time derivative signals to said grid controlled rectifier means, a reversible motor connected to be operated from the outputs of said grid controlled rectifier means, a controlled object driven from said motor, and a follow-up connection between said motor and a member of said transmitter-receiver arrangement whose position aflects the output voltage thereof.

10. In a positional controrsystem of the character described, controlling and controlled objects, transmitter-receiver means for producing a variable alternating potential signal responsive to the relative displacement of said objects, said signal reversing in phase with reversal of the direction of relative displacement of said objects, dual transformer and connected rectifier means, means for supplying a reference voltage to said dual transformer means, said signal potential being additive to said reference voltage in one of said transformer means and subtractive in the other, depending upon the direction of such relative displacement or the objects, said rectifier means converting said signal voltage into a direct current signal potential of reversible polarity also depending on the direction of the relative displacement of said objects, a combined amplifier and modulator for amplifying said signal potential and for reconverting the same into a reversible phase alternating potential, means including a transformer in the output circuit of said amplifier-modulator for producing a time derivative potential of said displacement potential for combination with the latter, grid controlled rectifier means connected for receiving said potentials, motive means controlled from said rectifier means and connected for driving said controlled object, and a follow-back connection between said motive means and said transmitter-receiver means.

11. A positional control system as defined in claim 10, wherein said combined amplifier-modulator and its output circuit transformer are pro vided with means for by-passing circulating capacity currents to thereby prevent such currents from adversely afiecting said grid controlled rectifier means and hence the sensitivity of the systern.

FRANCIS L. MOSELEY.

ERIC J. ISBISTER.

HAYES B. STEINHAUSER.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Davis Oct. 23, 1934 

